1. Field of the Invention
The present invention relates to solar energy technology and, more particularly, to solar radiation concentrators.
2. Description of the Prior Art
Solar energy technology concentrators of over two meters in diameter are made composite due to technological difficulties in approximating the rated paraboloidal or other surface with individual mirors-facets having different geometry than the reflecting surface.
Known in the art is a solar radiation concentrator (U.S. Pat. No. 4,295,709, U.S. Class 350-292), which includes a supporting frame made in the form of a paraboloid of revolution which by the accuracy of manufacture approximates to the maximum extent the rated surface on which flat mirrors are secured whose surface approximates the paraboloidal surface. With such an approximation of the paraboloidal surface the number N facets is of the same order as the coefficient C of concentration in the focal point. However, the manufacture of a true paraboloidal supporting frame and the assembly of a large quantity of facets thereupon are labour-consuming and entail sizable manufacturing costs despite apparent simplicity.
Also known in the art is a solar radiation concentrator (French Application No. 2477725, IPC.sup.3 G 02 B 5/10, 1981), which inclues a supporting frame with facets mounted thereupon which have a spherical reflecting surface with equal radii of curvature in every annular zone. The supporting frame is made in the form of annular zones displaced relative to the apex so as to make it possible to adjust the position of the focal image of a radiation source from all annular zones in the focal plane of the concentrator, diminishing the impact of spherical aberration of the reflected beams on the degree of energy concentration in the focal spot. However, with this arrangement of facets on the supporting frame, it is impossible to eliminate the impact of angular aberrations caused by a large angle of incidence between the normal to the facet and the incident beams parallel to the concentrator optical axis which decreases the degree of energy concentration in the focal spot. To attain the degree of concentration approximating that of an equivalent paraboloidal concentrator, there arises a necessity of reducing the radial size of facets and their number in annular zones.
Also known in the art is a solar radiation concentrator (cf. "Geliotekhnika", No. 1, 1976, pp. 33-39, FIGS. 1 and 2 on page 34) whose reflecting surface is composed of spherical annular zones arranged on the supporting frame, whose radii of curvature are optimized versus the angular coordinates of the centres of zones on the supporting frame with a view to attaining the maximum concentration. And radii R of curvature of the spherical facets in the zones satisfy the condition R.sub.s &lt;R&lt;R.sub.m, where R.sub.m and R.sub.s are meridional and tangential radii of curvature of the rated paraboloidal surface, respectively, in the point of tangency of the apex of the spherical facets.
With such an optical diagram of the concentrator one may achieve concentrations corresponding to that attained by the paraboloid of revolution, whose optical accuracy .delta. (in angular minutes) of fabrication is equivalent to the optical accuracy .delta. of the concentrator with spherical annular zones. However, a great number of spherical annular zones with different types of radii R of curvature and a large quantity of facets of the same type in the zone are needed to achieve the said concentration which increases the concentrator cost.
Also known in the art is an optical diagram of the concentrator (cf. the journal `Solar Energy`, No. 4, 1963, pp. 162-164, FIG. 1), whose reflecting surface is composed of toroidal facets arranged in annular rows on the paraboloidal supporting frame. The toroidal zone is obtained by turning the arc with a radius R equal to the meridional radius R.sub.m of curvature in the point of tangency of the arc with the rated generatrix of the paraboloid of revolution, the centres of toroidal zones lying in the same point of the generatrix of the paraboloid surface which is approximated by the said toroidal facets. In this case, of all the incident beams parallel to the optical axis of the concentrator it is only one beam falling on the point of tangency of the arc with the rated generatrix of the paraboloid that is reflected to the projected focus. Such a synthesis of the concentrator reflecting surface enables one to reduce the quantity of toroidal annular zones and increase the degree of concentration by regulating the orientation of individual facets on the paraboloidal supporting frame. However, the toroidal zones, obtained through this method and arranged with the centres on the generatrix of the rated parabola with the regulation of the facet orientation in annular zones, bring about errors both in the radial and tangential sections decreasing the maximum attainable degree C of concentration in the concentrator focal spot.
Besides, the regulation of the facet orientation in the paraboloidal supporting frame gives rise to aperture breaks in the radial section, and in the tangential section leads to fractures in the reflecting surface of the annular zones. Thus, the solar radiation falling on the concentrator is not fully utilized.